Operando chemo-mechanical evolution in LiNi0.8Co0.1Mn0.1O2 cathodes

ABSTRACT Ni-rich LiNixCoyMnzO2 (NCMxyz, x + y + z = 1, x ≥ 0.8) layered oxide materials are considered the main cathode materials for high-energy-density Li-ion batteries. However, the endless cracking of polycrystalline NCM materials caused by stress accelerates the loss of active materials and electrolyte decomposition, limiting the cycle life. Hence, understanding the chemo-mechanical evolution during (de)lithiation of NCM materials is crucial to performance improvement. In this work, an optical fiber with με resolution is designed to in operando detect the stress evolution of a polycrystalline LiNi0.8Co0.1Mn0.1O2 (P-NCM811) cathode during cycling. By integrating the sensor inside the cathode, the stress variation of P-NCM811 is completely transferred to the optical fiber. We find that the anisotropy of primary particles leads to the appearance of structural stress, inducing the formation of microcracks in polycrystalline particles, which is the main reason for capacity decay. The isotropy of primary particles reduces the structural stress of polycrystalline particles, eliminating the generation of microcracks. Accordingly, the P-NCM811 with an ordered arrangement structure delivered high electrochemical performance with capacity retention of 82% over 500 cycles. This work provides a brand-new perspective with regard to understanding the operando chemo-mechanical evolution of NCM materials during battery operation, and guides the design of electrode materials for rechargeable batteries.

The FBG sensor consists of cladding and core, while having a grating at a specific location.The diameter of cladding and core were 125 μm and 25 μm, respectively.
The grating is inscribed on the core at any location with the length of 5 mm.When the light travels through the core, the grating can reflect the specific wavelength of light, which called Bragg wavelength and was defined as   = 2 eff .Therefore, the value of Bragg wavelength depends on n eff (the effective refractive index) and Λ (the grating period).Owing to the elastic-optical effect and thermos-optical effect, the changes of temperature and strain appeared in the surrounding can alter either n eff or Λ, which will be visualized as the shift of Bragg wavelength.[1,2] Figure S2.The digital images of the pouch cell with an implanted FBG.
According to the experimental section, the electrode with an implanted FBG was prepared.Subsequently, the electrode was cut to be appropriate size by a knife without damaging the FBG sensor.And then the electrode and lithium foil were stacked up and down, separated by two fiberglass papers.After filling with the electrolyte, the pouch cell was vacuum sealed.Ultimately, connect the FBG sensor to the jumper so that it could be connect to the optical interrogator.During battery assembly, the FBG sensor was placed on the surface of the electrode, the specific operation can be referred to our previous work.Due to external applied pressure, a shallow trench was formed on the electrode in the region where the FBG sensor was.According to our previous work, the thickening and thinning of the electrode caused the FBG to lengthen and shorten.Hence, the stress evolution can be seen in Figure S2c.However, the slope of the stress kept almost unchanged during charge or discharge, which cannot reflect the complex phase transition of materials.During battery assembly, the FBG sensor was placed on the surface of the electrode and fixed on the aluminum plastic film by the tab film, the specific operation can be referred to our previous work.[3] During stress measurement, the constant pressure was applied on the pouch cell and both the electrode and optical fiber were pre-tightened, leading that the grating was firmly pressed onto the electrode.During delithiation of active particles, the volume reduction of active particles caused the electrode to thin, alleviating the stress state on the optical fiber.Hence, the optical fiber was stretched and shrunk repeatedly during repeated cycle, reflecting the stress evolution at electrode level.Following the method of experimental section, the pouch cell was prepared to be tested.The FBG sensor was implanted into the electrode and the electrode was not roll-pressed, shown in Figure 1d and 1e.Since the electrode was not roll-pressed, the holes could balance the volume changes of the NCM materials.Therefore, the stress changes were not periodic and remained basically constant.This configuration made it difficult to monitor the stress evolution.According to the ultrasonic transmission images, the FBG sensor had little effect on the wettability of the pouch cell.As shown in Figure S6c and S6e, the peak-to-peak value are equal to 1.7-1.8V.As for the pouch cell with an FBG sensor, the peak-to-peak value of the FBG sensor is slightly lower (point 2) than other locations (point 1).Such a small difference does not have any effect on battery performance.Due to the strictly controlled temperature, it can be found that the temperature barely changed and only a slight flutter occurred.Removing the interference of temperature, the stress evolution can be obtained (Figure 2c).All the subsequent stress evolution is obtained in this way, and the temperature signal is not be shown later.At a rate of 0.1C, the capacity of P-NCM811 materials is almost not reduced, indicating the good chemical reversibility.Accordingly, the stress evolution also had periodicity and swung with voltage.Meanwhile, the trend of stress evolution is similar of each cycle, exhibiting the good mechanical reversibility.At a higher rate of 0.2C, P-NCM811 materials delivered lower capacity of 192.6 mAh g -1 .Accordingly, due to capacity-dependent chemical strain, the minimum value of Δσ was also reduced.However, the trend of stress evolution has not changed and the abnormal increasing of stress during charge still exist.At a rate of 0.1C, the capacity of S-NCM811 materials is almost not reduced, indicating the good chemical reversibility.Accordingly, the stress evolution also had periodicity and swung with voltage.Meanwhile, the trend of stress evolution is similar of each cycle, exhibiting the good mechanical reversibility.Due to the effect of polarization, Δσ remained unchanged at voltage less than 3.6 V, meaning that no chemical reactions took place.When they were charged to 3.7V, the Δσ of these two materials began to differ, which was called stress anomaly interval.Although the peak shift of (003) still increased and then decreases, the amplitude of changes was only 0.89°, which is smaller than that of P-NCM811 materials.Hence, the variation of c-axis parameter can be smaller, resulting in smaller changes of crystal shape.

Figure S1 .
Figure S1.The work principle of FBG sensors.(a) The schematic diagram of sensing principle.(b) The top-view images of The FBG sensor.

Figure S3 .
Figure S3.The stress evolution of the NCM811 electrode with an attached FBG sensor.(a) The schematic diagram of the cell assembly.The FBG sensor was placed on the surface of the NCM811 electrode.(b) The top-view images of the NCM811 electrode where the FBG sensor was.(c) Galvanostatic cycling of Li||NCM811 pouch cell, along with the stress evolution measured by the FBG sensor.

Figure S4 .
Figure S4.Schematic illustration of the attached FBG sensor and its surroundings.

Figure S5 .
Figure S5.Cross-section images of the electrode.(a) Without being roll-pressed, the electrode was porous.(b) After being roll-pressed, the electrode became dense.

Figure S6 .
Figure S6.The stress evolution of the unroll-pressed NCM811 electrode with an implanted FBG sensor.(a) The schematic diagram of the cell assembly.The FBG sensor was implanted into the NCM811 electrode, and the electrode was not roll-pressed.(b) Galvanostatic cycling of Li||NCM811 pouch cell, along with the stress evolution measured by the FBG sensor.

Figure S7 .
Figure S7.The ultrasonic transmission images of the pouch cell and the corresponding ultrasonic waves.(a, c, e) The ultrasonic transmission images of the pouch cell without an FBG (a) and the corresponding ultrasonic waves of two points (c, e).(b, d, f) The ultrasonic transmission images of the pouch cell with an FBG (b)and the corresponding ultrasonic waves of two points (d, f).Therein, point 2 is located at the FBG sensor.

Figure S9 .
Figure S9.The measured wavelength shift of these two FBG sensors.The first FBG sensor was implanted into the electrode, which was used to obtain the stress information.Another FBG sensor was loosely placed on the pouch cell, which was used to measure temperature.

Figure S11 .
Figure S11.Stress evolution of P-NCM811 materials at 0.1C.(a) Corresponding capacity profile of P-NCM811 materials at 0.1C for 3 cycles.(b) The stress evolution of P-NCM materials, with the corresponding voltage curves for 3 cycles.

Figure S12 .
Figure S12.Stress evolution of P-NCM811 materials at 0.2C.(a) Corresponding capacity profile of P-NCM811 materials at 0.2C.(b) The stress evolution of P-NCM materials, with the corresponding voltage curves.

Figure S13 .
Figure S13.2D stack-view of the reflected spectra given by the FBG sensor implanted into the S-NCM811 electrode during cycling.

Figure S14 .
Figure S14.Stress evolution of S-NCM811 materials at 0.1C.(a) Corresponding capacity profile of S-NCM811 materials at 0.1C for 3 cycles.(b) The stress evolution of S-NCM materials, with the corresponding voltage curves for 3 cycles.

Figure S15 .
Figure S15.Stress evolution of S-NCM811 materials at 0.2C.(a) Corresponding capacity profile of S-NCM811 materials at 0.2C.(b) The stress evolution of S-NCM811 materials, with the corresponding voltage curves.

Figure S17 .
Figure S17.The stress evolution at the high voltage of 3-4.6 V. (a) The stress evolution of P-NCM811 materials, with the corresponding voltage curves.(b) Voltage-resolved dσ/dV profile together with the dQ/dV plot of P-NCM811 materials.(c) The stress evolution of S-NCM811 materials, with the corresponding voltage curves.(d) Voltage-resolved dσ/dV profile together with the dQ/dV plot at the high voltage of S-NCM811 materials.

Figure S18 .
Figure S18.The variation of a-axis parameter for P-NCM811 and S-NCM811 during charging process.

Figure S19 .
Figure S19.EBSD results of S-NCM811.(a) SEM images of cross-sections of S-NCM811 particles.(b) EBSD images and crystal plane distribution of S-NCM811 particles.

Figure S21 .
Figure S21.The characterization of OAS-NCM811 materials.(a-b) The scanning electron images of Gd-doped P-NCM811 materials with the corresponding elemental mapping of Ni, Co Mn and Gd.(c) XRD results of OAS-NCM811.

Figure S22 .
Figure S22.The related results of stress evolution of OAS-NCM811 materials.(a) 2D stack-view of the reflected spectra given by the FBG sensor implanted into the OAS-NCM811 electrode during cycling.(b) Voltage-resolved dσ/dV profile together with the dQ/dV plot of OAS-NCM811 materials.

Figure S23 .
Figure S23.The related results of stress evolution of OAS-NCM811 materials.(a) The stress evolution of OAS-NCM materials, with the corresponding voltage curves for 3 cycles.(b) The stress evolution of OAS-NCM811 materials at 0.2C, with the corresponding voltage curves.

Figure S24 .
Figure S24.The stress evolution at the high voltage of 3-4.6 V for OAS-NCM811 materials.(a) The stress evolution with the corresponding voltage curves.(b)Voltage-resolved dσ/dV profile together with the dQ/dV plot.

Figure S26 .
Figure S26.Cross-section of OAS-NCM811 particles before cycling (a) and after